Conventional p-n-p or n-p-n bipolar type phototransistors which have a uniform or gradually-graded base structure have defects of low sensitivity and low operating speed because of their high base resistance and large collector-base and emitter-base capacitances.
FIG. 1 shows an example of the conventional bipolar phototransistor, 4 being an emitter electrode, 1 an emitter region, 5 a base electrode, 2 a base region, 6 a collector electrode and 3 a collector region. 10 is a light input. The base electrode 5 may be made floating in some cases. 7 is an insulating material such as an SiO.sub.2 film or the like.
As shown in FIG. 1, in the portion of the base region 2, the distributed base resistance R.sub.1 between the emitter junction and the collector junction is high, because the emitter and collector are low in impurity density. Also, the distributed base lead-out resistance R.sub.2 is high, because the base is low in impurity density and small in base thickness, and further, there are other parasitic resistances. Therefore, the value of the base resistance of the bipolar phototransistor, including these parasitic resistances, is very large.
The ratio of the density, .DELTA.In, of an amplified electron current flowing between the emitter and collector of the phototransistor of the n-p-n structure to the total current density I by incident light, that is, the current amplification factor, .DELTA.In/I, is given by the following equation in the case where the incident light intensity is very low and a dark current component is also small. ##EQU1##
Here, D.sub.n and D.sub.p are diffusion coefficients of electrons and holes, L.sub.p is the diffusion length of holes, W.sub.b is the thickness of the base, n.sub.e is the impurity density of the emitter and P.sub.b is the impurity density of the base.
Eq. (1) is just the injection ratio of the emitter junction, I.sub.n /I.sub.p, and this means that the higher the injection ratio of the transistor is, the more its current amplification factor increases. I.sub.n is the total emitter current and I.sub.p a hole current.
To increase the current amplification factor of the conventional bipolar base phototransistor, it is required to raise the impurity concentration of the emitter region, and to reduce the impurity concentration of the base region and to decrease the thickness of the base region. However, the reduction of the impurity concentration and the thickness of the base region causes an increase in the parasitic base resistance. Therefore, this modification is not desirable.
Next, consider the operating speed of the phototransistor. The time constant for its rise and fall is given substantially by the following equation: ##EQU2##
Here, +.PHI..sub.eb is the diffusion potential between the base and the emitter. The time constant decreases with a decrease in L.sub.p /D.sub.p, but the current amplification factor diminishes according to Eq. (1). A decrease in the thickness W.sub.b of the base will decrease the time constant and increase the current amplification factor. The value of +.phi..sub.eb is the diffusion potential which is dependent upon the impurity densities of the base and the emitter, and the reduction of this diffusion potential will cause a decrease in the time constant. Further, the value of +.phi..sub.eb decreases with a decrease in n.sub.e or P.sub.b, but in order to prevent the reduction of the current amplification factor, it is necessary only to decrease P.sub.b.
For the reasons given above, in order to raise the current amplification factor of such a conventional phototransistor of bipolar structure to speed up its operation, there is no choice but to decrease the thickness W.sub.b of the base to thereby reduce its impurity density. As described previously, however, this will increase the base resistance to impose limitations on the performance of the transistor, providing only very unsatisfactory results. At present, a p-i-n photodiode, an avalanche photodiode and so forth are widely used as photosensing elements, but the diode with two terminals is defective in that it is not sufficiently isolated from the succeeding stage. The avalanche photodiode calls for a relatively high voltage (several tens of volts) and has a serious drawback of some amount of avalanche multiplication noise.
Accordingly, the present invention has for its object the providing of a high-speed and high-sensitivity phototransistor which is free from the abovesaid defects.